Metal Requirements for Building Electrical Grid Systems of Global Wind Power and Utility-Scale Solar Photovoltaic until 2050

Wind and solar photovoltaic (PV) power form vital parts of the energy transition toward renewable energy systems. The rapid development of these two renewables represents an enormous infrastructure construction task including both power generation and its associated electrical grid systems, which will generate demand for metal resources. However, most research on material demands has focused on their power generation systems (wind turbines and PV panels), and few have studied the associated electrical grid systems. Here, we estimate the global metal demands for electrical grid systems associated with wind and utility-scale PV power by 2050, using dynamic material flow analysis based on International Energy Agency’s energy scenarios and the typical engineering parameters of transmission grids. Results show that the associated electrical grids require large quantities of metals: 27–81 Mt of copper cumulatively, followed by 20–67 Mt of steel and 11–31 Mt of aluminum. Electrical grids built for solar PV have the largest metal demand, followed by offshore and onshore wind. Power cables are the most metal-consuming electrical components compared to substations and transformers. We also discuss the decommissioning issue of electrical grids and their recovery potential. This study would deepen the understanding of the nexus between renewable energy, grid infrastructure, and metal resources.


Inter-array Grids
The layout and topology of inter-array grids for wind and utility-scale solar PV power projects are complex. Taking offshore wind farms as an example, to minimize the length of cable between turbines and the total cable cost, and to reduce cable failures, various layout models have been developed to optimize the array cable routing, such as string structure, radial structure and closed-loop structure [15][16][17] . We have used an empirical formula 18,19 to estimate the array cable length of a typical wind farm 18,19 . We assume that the inter-array cables of both offshore and onshore wind farms follow the same empirical formula, as follows: (eq. S1) Where refers to the required length (unit: km) of inter-array cables and refers to the capacity (unit: MW) of wind farms. Both 0.00067 and 14.6 are constants that have been empirically derived from the layout of existing parks 18 . For the inter-array cable length of utility-scale solar PV, the cable length coefficient in recent reports [20][21][22][23] , averaging about 1.9km/MW, is adopted to estimate the total demand for inter-array cables (Table S1).

Transformers & Substations
The rated power size and number of transformers and other electrical equipment in a substation (e.g. switchgear, circuit breaker), also varies with the voltage and power output required for different projects. To simplify the electrical system model, here we refer to the general design principles of wind or solar PV power plant circuits 24,25 : there are two main transformers and other substation equipment for each project, one is for daily use and the other is for backup.
The rated power of transformers installed in these power plants is roughly proportional to the installed capacity of the power projects. Based on this assumption, the transformer S6 specifications installed in wind/solar PV power plants of different types and years are estimated, as detailed in the following section 1.4.2. Value of DC cable adopted in the study 1

.5 (main DC cable)
The shorter the cable, the less energy is lost. The related development trend is to optimize the topology of cabling and shorten the length of cables. To reflect future trends, we use the smallest value available. 23

(AC cable)
The value is derived from the paper in 2009.

(AC cable)
The latest data available is more than ten years ago. In order to reflect technological changes, we assume that the current AC cable coefficient of solar PV farms has also dropped by 40% compared to before based on the trend of the DC cables over time.

Total length 1.92 (DC + AC cable)
In general, there are three types of cables used in a PV system: DC string cables, DC main cables, and solar AC cables. DC string cables are preassembled into the panels by the PV panel manufacturer, so they usually cannot be changed. While DC main cables and solar AC cables both are larger power collector cables, which are installed in the transmission grid development stage. So we only consider the latter two cables.

Metal composition and intensities for transmission lines
Typical power cables and transformers used in offshore wind, on-shore wind and utility-scale solar PV projects have their own characteristics. Even the power grid with the same technology has its own characteristics in the selection of infield cables and outfield cables. This section presents the material intensities of electrical grid components for different energy technologies and different parts of the electrical grids. literature, technical reports, product manuals, and our assumptions and estimates. We elaborated on this in the following section.
In addition, we assume there will be no revolutionary breakthroughs in transmission technology in the future. It is generally believed that with the continuous increase of transmission distance and output power, high-voltage DC and flexible DC transmission will be more common than AC transmission 26 . However, such changes generally have no significant impact on the cable metal composition. Besides, other power transmission technologies such as high-temperature superconductors are still also on the rise, and it is expected to be difficult to widely apply in the short term due to their high costs and technical challenges 27,28 . Accordingly, the impact of transmission technology on the demand for metals thus is not considered in this study.
Furthermore, some studies also have estimated the material intensities of power cables and transformers for the whole electrical grid networks. By comparison, it is found that the material density of cables or transformers used in these studies is higher than that in this study. The difference is understandable. This is because the electrical grids concerned in this study are the inter-array and export transmission lines for wind and photovoltaic renewable energy projects.
The smaller grid transmission capacity of individual renewable energy projects tends to correspond to smaller conductor diameters and thus smaller average material intensities.
However, current studies do not subdivide the power grids corresponding to different technical categories but focus on all power grids, which may include dedicated transmission lines (e.g. China's West-to-East Power transmission project). This makes the load capacity of the hypothetical circuit larger, and thus the larger corresponding average material intensities. S10

Metal intensities of transmission cables and lines
Offshore Offshore wind farm cable elements include array cables that connect each wind turbine and bring power from turbines to a substation platform and export cable that transmits power from substation to landfall. A submarine power cable is a transmission cable for carrying electric power below the surface of the water, which is widely used in offshore wind farms. Compared to overhead and landlines, submarine power cables are usually equipped with single or double armor (e.g. stainless steel wire armor) to protect cables from seawater corrosion and external impact, but this also results in a larger diameter and heavier mass of submarine cables. Currently, due to the diverse application, there is a large number of submarine power cables available on the market, with various conductor choices, shapes, sizes, lengths, etc. The choice of cable used as array cable and export cable is usually very different because of the difference in capacity load, rated voltage, cross-sectional area, cable length, and other aspects, and this also leads to the difference in their metal intensities. To get a general idea about the different characteristics of these two types of power cables, as well as their most generic metal intensities, we discuss the most important properties of these two types of submarine power cables.

Array cables:
The infield voltage values of offshore wind parks that have been built and operated generally range from 20 kV to 35 kV 29-32 . Among them, 33 kV (rated at 36 kV) is the most commonly used voltage level in inter-array grids, which is called standard voltage in some reports [32][33][34] . Meanwhile, since the average capacity of wind farms and the average single capacity of each wind turbine are going to increase, 66 kV infield-array systems with higher voltage level is expected to be applied for future new projects. Operating at this increased voltage could result in lower power losses, fewer array cabling, and potential cost savings that come with it, and this technology is expected to be applied in more new-built offshore wind projects 32,[35][36][37][38] . In addition, some offshore wind projects use an infield voltage of 45 kV for S11 their inter-array grids. Therefore, here we assume that the grid voltage in the offshore wind farm will be within 33-66 kV range for the next 30 years.
After the indicative array power systems were established, the most representative corresponding cable conductors and the associated conductor cross-sectional areas were also determined. Regarding the choice of conductor material for submarine cables, generally, copper conductor cable is most commonly used for connecting offshore wind farms 39 . This is because copper conductors offer a more stable structure in the seabed and longer life cycles in harsh operating conditions. At the same time, due to lower raw material costs and installation costs, some utilities see the economic opportunities of using aluminum as submarine cable conductors and have begun exploring the potential of aluminum submarine cables 40,41 . However, It is still not clear whether aluminum submarine cables will be used widely for future offshore wind projects, and some reports suggest that copper submarine cables are likely to remain dominant in the future 42 . Here, we used the 2016 penetration rate of aluminum in power cables (approximately 16%) as the proportion of aluminum submarine cables used in offshore wind power projects in the following decades 43 , and the other 84% of submarine cables are copper conductors. In addition to conductors, the metal parts of the submarine power cable may also be sheath and armor, which are added to prevent water ingression and protect the cable from mechanical loading as well as taking care of the tension stability respectively 44,45 . The armor consists of metal sire, commonly galvanized steel; the sheath material can be lead, copper, copper, and sometimes polymer. Because of the limited data, we assumed all submarine power cables in this study are made of the lead sheath and steel armor.
In terms of conductor size, two other main factors affecting the metal intensity of the cable need to be considered. One is the number of conductor cores per cable, and the other is the crosssectional area of the conductor, which are also indispensable parameters for cable specification.
Currently, the majority of offshore wind power projects use three-core AC (alternating current) cables in the infield circuit and rarely use single-core cables. There is no other related technological innovation to date, it is expected that three-core cables will still occupy the main market of array submarine power cables in the future. The array submarine power cable usually has multiple cross-sectional areas along the route of array electrical systems 46 . This is because array submarine cables in different parts of the infield electrical system carry different amounts of electricity. Generally the closer the array cable is to the substation, the more power it needs Taking these factors into consideration, we collect the data on submarine cables from publicly disclosed information from cable manufacturers or offshore wind project operators, and relevant literature reports. Then we take their average value as the metal intensity of infield cables of offshore wind farms in the next thirty years, as shown below.   Export cable: export submarine power cables that transmit electricity from the offshore substation to land are of similar design with the array submarine power cables but for higher voltage to carry more energies. This also causes a much greater conductor diameter of the export cable than that of the array cable. The voltage level of export submarine power cable for offshore HVAC (High Voltage Alternating Current) transmission is typically between 100 and 320 kV 42,45,59 . When the distance to shore gets longer, HVDC (High Voltage Direct Current) transmission technology and associated submarine power cables are preferred due to lower electrical losses and lower costs 60 . The voltage level of HVDC cables for export cabling of OWFs can be up to 525 kV 59 . In terms of conductor material selection, we assume that the penetration rate of conductor materials of the export cable is the same as that of array cables: copper conductor submarine cables and aluminum conductor cables account for 84% and 16% of the submarine power cable market respectively. Regarding conductor size, the AC (Alternating Current) cable commonly has 3-core conductors, whereas the DC (Director S19 Current) cable is 1-core conductors 45 . Due to the lack of relevant data, here we assume that the possibility of using AC and DC technologies in the export electrical system of OWFs in the future is equal, which means that the probability of laying a 1-kilometer export submarine power cab with AC and DC cables is 50% respectively. The cross-sectional areas of submarine power cables for export cabling of OWFs commonly range from 400 to 1200 mm2 61 , and in some cases, it can be up to 1400 mm 257 . In combination with the above discussion, the metal densities of typical export submarine power cables are estimated as below.

Onshore
Similar to the power transmission system of offshore wind farms, power transmission systems of onshore wind farms mainly consist of infield-array grid systems and export grid systems. The inter-array grid system connects each wind turbine and collects electricity from wind turbines to the substation, and export grid systems transport power from the onshore wind farms to the main transmission network or directly to the nearest distribution network. Generally, the cable used for the infield-array grid system of onshore wind farms is underground cable, while the cable for export grid system can be either S24 underground cable or overhead cables. The underground cable is buried in the ground and equipped with lead sheet and armoring, which protects against moisture and mechanical injury. While overhead transmission cable uses bare conductors and these conductors are placed at a height from the ground.
The choice of cable for inter-array grid systems and export grid systems of onshore wind farms also varies with different projects, depending on expected nominal voltage, power load, investment cost, etc., but they do also have their characteristics. We estimated the metal intensities of these cables based on their features as follows.

Array cable:
We adopt similar analysis steps as for offshore wind farm cables to determine the generic characteristic parameters of different parts of the cable system, then estimate the typical cable intensities for array cable as well as export cable for onshore wind farms based on these characteristics.
The voltage level of the array cable system of an onshore wind farm is commonly in the range of 10 to 66 kV 64 . The conductor material of these array cables could be copper or aluminum.  Table S11. Metal intensities of typical array underground cables for onshore wind farms.
As discussed above, we assume that copper conductors will account for 84% of the submarine power cable market and aluminum conductor cable for 16% in the future. Then we use the weighted average of the material densities of these two types of array cables to represent the amount of material required for each kilometer of array power cables in the coming decades.

Utility-scale Solar PV
Similar to wind power farms, the cable transmission system of utility solar PV plants includes in-field array cable for the connection between the PV components, and a dedicated export S35 cable line (usually called a generation tie or "gen-tie") for transferring output electricity to the existing substation or main grid. According to the characteristics of utility solar PV plants, we discussed its typical cable intensity. Same as other power cables we discussed before, we again apply the same assumption of the market penetration rate of copper and aluminum conductors (84% and 16% respectively). Based on these characteristics of solar cables, the metal intensity of typical solar PV cables are estimated as follows.   to the main grid. These export cables can be implemented using underground cables or overhead cables depending on specific projects, which is the same as that of onshore wind farms. Here we use the metal intensity of the export cable of onshore wind farms to represent the metal intensity of the export cable of solar PV plants (see Table 14).

Transformers and substation intensity
Power grids usually operate at much higher voltages in the order of tens or hundreds of thousands of volts, transformers are incorporated in renewable energy projects to deliver the  In addition to cables and transformers, the power transmission system also needs other auxiliary power equipment, such as switches, circuit breakers, etc. The equipment is often placed in substations for unified management. When planning renewable power projects, the construction of substations is also considered. The substation further boosts the power collected from renewable power plants for subsequent transmission to the main grid. Substations are also very diverse in size and complexity, and it is impossible to examine the very detailed layout information for different renewable power projects. Here we summarize and estimate the typical metal intensity of some important auxiliary power equipment used in substations, as shown in Table S20. In addition, due to the limited data, we assume that the substations supporting all kinds of renewable energy power plants are the same.

Sensitivity analysis
Our model outputs are based on a set of assumptions on variables: the lifetime distribution of power projects, the metal intensities for each component of e, array cable length for individual projects, and distance to main grids. The purpose is not to use our model to predict the future, but rather to explore quite different future scenarios of global development of electricity transmission infrastructure triggered by wind and solar PV generation and thus of metal requirement, and to understand key factors influencing the amount of required metal. A sensitivity analysis is thus performed for understanding where the major uncertainty may arise, S45 as well as assessing the impact of modeling assumptions on the simulation outcomes. We assess the impacts of lifetime parameters on outputs by applying a different set of normal distribution parameters. We also compare the headline outputs to alternative simulation outputs where the metal content of copper is assumed to be 10% less than the current level. In addition, we examine the effect of the cabling optimization of the inter-grids and project site selection on the amount of metal required by assuming that the array cable length of a project decreases by 10% and the distance to main grids increases by 10%. All alternative simulation processes are placed in the SDS scenario and only copper is tested.

S55
Copper flows contained in electrical grid systems for wind projects in the SDS scenario are selected to demonstrate the impacts of main uncertainties on modeling output. Figure 8 presents the results of model sensitivity analysis for onshore wind projects (see Figure S9 for offshore transmission lines). A longer average lifetime of renewable projects would reduce both the copper demand and outflows per period, as is expected. If the average lifetime of Normal lifetime distribution is extended to 25 years, copper demand per period will decrease to 685 kt per period by 2050, which is a about 18% drop compared to the original estimate. Meantime, as the average lifetime increases, the copper outflow also decreases, which keeps the difference between copper inflow and outflow almost unchanged. Besides, the smaller standard deviation parameter somewhat delays the required copper inflow, but after 2036, the effect is almost negligible. All of the above suggests that extending the life of the renewable program through technological advances could reduce overall future metal material requirements for building their corresponding transmission lines despite the almost unchanged difference between inflow and outflow.
Metal contents also have a significant impact on metal demand for power transmission lines.
Grid operators have been trying to increase the share of cheaper aluminum conductors in submarine and underground power cables because of cost concerns. At the same time, the recent increasing use of DC power transmission systems will also reduce the metal content of power cables to a certain extent because compared to an AC power cable with a minimum of three wires, a DC power cable generally only uses two wires but has a greater power load capacity.
Both trends indicate that technological developments are likely to reduce the metal content of power transmission lines. Following this trend, for onshore wind projects, a 10% reduction in copper content in cables would reduce copper demand per period by about 6% by 2050. For offshore wind, the copper demand per period would also fall by 10%. This also reminds us that we should improve the quality of information and data related to the metal content of S56 transmission cables and substations in the future, which will have a positive impact on the accuracy of the estimates.
The cable layout of inter-array grids and distance to interconnection points are also key factors in estimating metal demands. In the case of wind farms, different cabling layouts of the interarray grid differ in total cable length, and thus power loss, generation efficiency, and costs, etc.
Many studies have been conducted to find the optimal cable arrangement and topology of the inter-array system to find the optimal balance between increasing the power generation efficiency and reducing the required cable length 17,79,80 . The sensitivity analysis shows that if the total inter-array cable length is reduced by 10%, the copper demand associated with wind power projects would be reduced by 6% (onshore) and 7% (offshore) respectively. In addition, although the length of the export power cable related to the distance from the main network is likely to increase in the future, the increase in this parameter has little impact on the copper demand. These results further emphasize the importance of infield cable optimization from the perspective of metal resource demand.
The length coefficient of inter-array cable is a key consumption for estimating metal demand for utility-scale solar PV technology. The sensitivity analysis shows that if the length coefficient is reduced by 10%, the copper demand with solar PV projects would reduce by about 9%. This result also shows the robustness of the model.